Optical transmission apparatus

An optical transmission apparatus is disclosed which comprises a light emitting element for outputting an optical output in order to transmit data, a current modulation circuit, a bias current generating circuit and a resistor circuit connected in parallel with the emitting element. The current modulation circuit supplies a binary modulated current to the emitting element in response to an input binary data to the transmitted, the bias current generating circuit provides a bias current to the emitting element and the resisor circuit, whereby said bias current is kept higher than a specified level.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical transmission apparatus in which 
a laser diode (hereinafter called LD) is used as a luminous element for 
outputting an optical data signal. 
2. Description of the Prior Art 
FIG. 1 illustrates a circuit embodiment of an optical transmission 
apparatus according to the prior art which has been described for example 
in an article No. 2246 title "Optical Transmission Apparatus with 100 Mb/s 
Laser Diode Provided with Carrier Detection Circuit" reported at the 
General National Assembly of Society of Electronic Communication 1981. In 
FIG. 1, numeral 1 designates a transmission data input terminal to which 
transmission data is input, numeral 2 a modulator adapted to modulate a 
current to a binary current in accordance with the transmission data input 
to the input terminal, numeral 3 an LD as a luminous element which emits 
light in accordance with the binary current output from the modulator 2, 
numeral 4 a light receiving element adapted to receive a portion of the 
light emitted by the LD 3 and transduce, or convert the received optical 
signal to an electrical signal, numeral 5 a reference current source 
adapted to output a reference current, numeral 6 a current amplifier 
adapted to supply a drive bias current which is proportional to the 
difference between the reference current provided by the reference current 
source 5 and the signal current transduced by the light receiving element 
4, numeral 7 a bias power source for the light receiving element 4 and 
numeral 8 a capacitor connected in parallel with the light receiving 
element 4. 
FIG. 2 is a characteristic diagram illustrating P-I characteristics of the 
drive current for an LD vs the optical output of the LD. In FIG. 2, symbol 
A designates the P-I characteristic at a low temperature while symbol B 
designates the P-I characteristic at a high temperature. 
FIG. 3 illustrates an example of an embodiment of the current amplifier 6 
wherein numerals 61 through 63 designate transistors. 
Operation of the prior optical transmission apparatus as described above 
will next be explained. The transmission data input to the input terminal 
1 is input to the modulator 2. The modulator 2 generates a current 
corresponding to the transmission data and supplies it to the LD 3. The 
transmission data is also input to the reference current source 5 and the 
reference current I.sub.O which is obtained by adding a current 
proportional to the mark ratio of the transmission data and a constant 
current will be output therefrom. The portion of the optical output from 
the LD 3 is led to the light receiving element 4, so that the light 
receiving element current I.sub.PD flows. The differential current between 
the reference current I.sub.O from the source 5 and the current I.sub.PD 
flowing through the element 4 is input to the current amplifier 6 and 
after being amplified to a specified magnification .beta., it is supplied 
to the LD 3 as the bias current. The bias current always flows through the 
LD 3, and the LD illuminates when the current from the modulator 2 flows 
through the LD 3. As the optical output from the LD 3 becomes stronger, 
the current flowing through the light receiving element 4 will increase. 
Therefore the output current from the current amplifier 6 will be smaller 
whereby the optical output from the LD 3 will be reduced. In the case 
where that the optical output from the LD 3 is reduced, the optical output 
from LD 3 will be conversely increased due to a similar reason. 
Accordingly, the optical output may be substantially kept at a constant 
value owing to the negative feedback operation as described above. 
As far as the peak value of the optical signal at the time of transmission 
of the digital signal is concerned, the peak value P.sub.out of the 
optical output from the LD 3 may be expressed in the following equation: 
EQU P.sub.out =A(I.sub.B +I.sub.OP -I.sub.th) (1) 
provided, 
EQU I.sub.B =.beta.(I.sub.O -I.sub.PD) (2) 
EQU I.sub.PD =mDLP.sub.out ( 3) 
where 
I.sub.B =bias current (current output from the current amplifier) 
I.sub.OP =modulated current (current output from the modulator) 
I.sub.th =LD threshold current 
.beta.=amplification ratio of the current amplifier 
I.sub.PD =light receiving element current 
I.sub.O =reference current 
m=mark ratio of the digital signal (0&lt;m.ltoreq.1) 
L=current transducing ratio of optical output vs light receiving element 
A=LD current/optical transducing efficient 
D=pulse duty factor. 
From the foregoing equations (1), (2) and (3), the following equation may 
be derived: 
##EQU1## 
In the equation (4), in order that the optical output P.sub.out be constant 
regardless of mark ratio change M, the reference current I.sub.O must be 
controlled in accordance with the mark ratio. And supposing P.sub.out =K 
(constant), 
EQU I.sub.O =I.sub.O1 +mI.sub.O2 
provided, 
##EQU2## 
EQU I.sub.O2 =KDL 
As expressed by the foregoing equation (5), the reference current source 5 
will supply the current I.sub.O comprising a constant current I.sub.O1 and 
the current mI.sub.O2 proportional to the mark ratio, so that the optical 
output from the LD 3 may be constant regardless of the mark ratio. 
Operation of the optical transmission apparatus when the temperature has 
varied will next be explained. Supposing that the parameter which varies 
depending on the temperature is only the LD threshold current I.sub.th as 
illustrated in FIG. 2, the following equation may be derived from equation 
(4): 
##EQU3## 
If the value of the Fabry-Perot type LD of 1.3 .mu.m band is considered, 
values of the efficient A, current transducing ratio L and 
.differential.I.sub.th /.differential.t are almost as follows: 
EQU A=0.11(W/A) 
EQU L=0.14(A/W) 
##EQU4## 
Then supposing that the temperature range is -30.degree. C.-+85.degree. 
C., and the case of the optical output 1 mW at T=25.degree. C. is 
considered. In order that the optical output be less than 1 db with the 
respective mark ratio, the following equation may be derived from equation 
(6): 
##EQU5## 
From equation (7), .beta..gtoreq.17737 is produced, provided that the 
minimum value of the mark ratio is 1/8. 
As explained above, a very large value of the current amplification ratio 
.beta. of the current amplifier 6 is required, therefore such a Darlington 
type amplifier as shown in FIG. 3 is employed. Supposing that the current 
amplification ratios of the transistors 61 through 63 are respectively 
.beta..sub.1, .beta..sub.2 and .beta..sub.3, the current amplification 
ratio of the amplifier will be expressed as follows: 
EQU .beta.=.beta..sub.1 .multidot..beta..sub.2 .multidot..beta..sub.3( 8) 
In general, since a current amplification ratio of an npn transistor which 
may be obtained is more than 30, the current amplification ratio .beta. 
obtained by the current amplifier shown in FIG. 3 is more than 27,000. 
Accordingly, if the transistors 61 through 63 are in conductive condition, 
then the current amplifier 6 shown in FIG. 3 is supposed to have a 
sufficiently large current amplification ratio, so that an adequate APC 
(Automatic Power Control) characteristics may be obtained. 
Since the optical transmission apparatus according to the prior art is 
constituted as above explained, the bias current I.sub.B of the LD 3 is 
generally set below the threshold current I.sub.th. Accordingly, at a high 
temperature, even if the bias current I.sub.B1 is made large enough as 
seen in FIG. 2, the bias current I.sub.B2 may be almost zero at a low 
temperature as seen in FIG. 2. In this low temperature condition, the 
transistors 61-63 of the current amplifier shown in FIG. 3 are shut down, 
and the current amplification ratio of the amplifier will become so small 
that the amplification ratio required for keeping the peak value of the 
optical output constant relative to the mark ratio and the temperature 
fluctuation will no longer be secured and this reduction of the current 
amplification ratio will degrade the APC characteristics. 
Further, it is sometimes necessary to stop the optical output from the 
optical transmission apparatus in order to check interruption of a network 
by passing another optical signal through an optical fiber for the purpose 
of maintenance of an optical transmission apparatus. 
It also happens sometimes that the bias current flowing through the LD 3 
will be monitored for maintenance of the apparatus and when the bias 
current value becomes abnormal, it is detected to energize an alarm. 
In FIG. 4, there is illustrated an optical transmission apparatus according 
to the prior art which is provided with the function of stopping any 
optical output from the optical transmission apparatus and monitoring the 
bias current of the LD 3 as above-described. 
In FIG. 4, the circuits having the similar functions as those of the 
circuits shown in FIG. 1 are denoted by the same numerals. It is to be 
noted that the transmission apparatus shown in FIG. 4 executes 
transmission operation substantially in a similar manner as that shown in 
FIG. 1. In the apparatus shown in FIG. 4, the modulation circuit 2 
comprises transistors 21 and 22 forming a current switching circuit, a 
constant current source 23 and a switch 24 and is adapted to draw a pulse 
current, or binary current into the collector of the transistor 22 in 
response to the input data to the base of the transistor 21 whereby the LD 
3 generates the optical output in response to the pulse current by the aid 
of the bias current from the amplifier 6. The reference current source 5 
comprises a controllable constant current source 51 and a switch 52. The 
switches 24 and 52 are turned off by an inhibit signal supplied to an 
inhibit control terminal 9 in order to stop the optical output from the 
optical transmission apparatus. 
The apparatus shown in FIG. 4 is further provided with a monitoring circuit 
10 for monitoring the bias current. The monitoring circuit 10 comprises a 
resistor 101 for monitoring current connected between the cathode of the 
LD 3 and the output stage transistor of the current amplifier 6 and a 
buffer amplifier having a unitary gain which comprises resistors 102, 103, 
104 and 105 and an operational amplifier 106. The monitoring circuit 10 is 
adapted to generate the same voltage as that generated across the opposite 
ends of the resistor 101 to the output terminal 107 whereby the current 
flowing through the LD 3 and the amplifier 6 can be monitored. 
According to the apparatus shown in FIG. 4, when the optical output is 
prohibited by the inhibit signal from the terminal 9, the potential 
between the opposite ends of the resistor 101 in the monitoring circuit 10 
will be zero and the voltage between the anode and cathode terminals of 
the LD 3 will also be zero. Accordingly the common mode input voltage to 
the buffer amplifier will be equal to the potential at the anode of the LD 
3 whereby it will be equal to the potential of a power source V.sub.CC 
connected to the anode of the LD. Under this condition, the input voltage 
to the buffer amplifier will be outside the common mode operational range, 
and therefore the output terminal 107 may be dropped to the potential of a 
power source V.sub.EE connected to the operational amplifier 106. This may 
cause, therefore, the monitoring circuit not to operate properly and 
provide an erroneous alarm when the optical output is prohibited. 
SUMMARY OF THE INVENTION 
The present invention has been proposed to eliminate the problems as above 
explained. Accordingly an object of the present invention is to provide an 
optical transmission apparatus which provides excellent APC 
characteristics, not depending on the P-I characteristics of LD and any 
temperature fluctuation. 
Another object of the present invention is to provide an optical 
transmission apparatus which is capable of enhancing APC characteristics 
of LD as well as keeping an extinction ratio and the peak value of the 
optical output constant. 
A further object of the present invention is to provide an optical 
transmission apparatus which is prevented from providing any erroneous 
alarm when the optical output is prohibited. 
The first object of the present invention is achieved by an optical 
transmission apparatus which comprises a current modulation circuit 
adapted to supply a binary current to a light emitting element in 
accordance with an input binary transmission data, a bias current circuit 
adapted to supply a bias current to the emitting element, and a resistor 
circuit connected in parallel with the emitting element so that the bias 
current is kept higher than a specified value. 
The second object of the present invention is achieved by an optical 
transmission apparatus which comprises a current modulation circuit 
adapted to supply a binary current to a light emitting element in 
accordance with an input binary transmission data, a bias current circuit 
adapted to supply a bias current to the emitting element, a resistor 
circuit connected in parallel with the emitting element so that the bias 
current is kept higher than a specified value and a temperature 
compensation circuit for controlling the value (magnitude) of the binary 
current so as to be made larger as the temperature increases. 
The third object of the present invention is attained by connecting a 
constant current source, the value of which is less than an oscillation 
threshold current of an LD for outputting an optical output, in series to 
the LD in an optical transmission apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will now be explained by referring 
to FIG. 5. In FIG. 5, the same components as those shown in FIG. 1 are 
denoted by the same numerals. The optical transmission apparatus according 
to the embodiment is characterized in the provision of a resistor element 
11 connected in parallel to the light emitting element (LD) 3. 
Operation of the embodiment shown in FIG. 5 will next be explained. In 
general, a voltage VF in the forward direction of an LD is in the order of 
1.2 (V). Accordingly if the resistance value of the resistor 11 is assumed 
to be R.sub.B, the current I'.sub.B which flows into the current amplifier 
6 through the resistor 11 is expressed as follows: 
EQU I'.sub.B .perspectiveto.1.2/R.sub.B (9) 
Assuming R.sub.B =300 (.OMEGA.), then I'.sub.B =4 mA. This means that even 
in the worst case, the current above 4 mA may flow into the current 
amplifier 6, or there will never be any shut-down condition at the 
amplifier 6, whereby a high current amplification ratio .beta. may be 
assured. For the value of the resistor 11, an adequate value, which is 
much higher than the resistance R.sub.ON of the LD 3 when the LD 3 is in a 
ON condition, will be preferred. R.sub.ON is normally in the order of 5 
(.OMEGA.). If the value R.sub.B is more than 100 (.OMEGA.), the modulation 
current output from the modulator 2 may substantially flow to the LD 3, so 
that reduction of the modulation efficiency is negligible. 
FIG. 6 and FIG. 7 illustrate second and the third embodiments of the 
present invention. In FIGS. 6 and 7, numeral 11.sub.1 designates a 
variable resistor and numeral 12 designate a temperature sensor. In these 
embodiments, the stationary resistor 11 in the first embodiment has been 
replaced by the variable resistor 11.sub.1 the value of which will be 
controlled depending on the temperature by means of the sensor 12. Numeral 
11.sub.2 in FIG. 7 designates a stationary resistor which is adapted to 
prevent an excessive current from flowing when the resistance value of the 
variable resistor 11.sub.1 is very small depending on temperature and 
becomes substantially equal to the impedance of the LD 3. The value of the 
resistor 11.sub.2 is some hundred ohms. 
The variable resistor 11.sub.1 is formed for example of FET 111 as shown in 
FIG. 8. The temperature sensor 12 is constituted of a resistor 121, a 
negative characteristic thermistor 122 and a power source 123 as shown in 
FIG. 8. The resistance between the drain-source of FET 111 is in the range 
of 300-10 K.OMEGA. and variably controlled by the output of the 
temperature sensor 12. 
According to the second and third embodiments of the present invention 
since the variable resistor 11.sub.1 which is variable depending on the 
temperature is connected in parallel to the LD 3, the current 
amplification ratio of the current amplifier 6 may be kept at a higher 
level in the entire temperature range than such a resistor might be 
otherwise connected, so that excellent APC characteristics may be assured. 
In FIG. 9, there is illustrated a fourth embodiment of the present 
invention in which a modulated current temperature compensation circuit 13 
is added to the first embodiment. The compensation circuit is adapted to 
make the value (magnitude) of the output current, or binary current from 
the modulator 2 higher as the temperature increases and smaller as it 
decreases. 
The operational principle of the above-described compensation circuit 11 
will next be explained. 
As shown in FIG. 2, the current/optical transfer efficiency of an LD has a 
certain temperature characteristics and in the case of a Fabry-Perot type 
LD, such efficiency is varied in the order of 0.05 W/A at the temperature 
range of 0.degree.-70.degree. C. In FIG. 2, the binary current and the 
bias current have the values shown in Table 1 respectively. 
TABLE 1 
______________________________________ 
Modulated current 
Bias current 
______________________________________ 
At a low temperature 
I.sub.M1 0 
At a high temperature 
I.sub.M2 I.sub.B1 
______________________________________ 
Since the temperature compensation circuit 13 is adapted to vary the 
magnitude of the binary current depending on temperature, the drive 
current I.sub.D (=I.sub.B +I.sub.M) of the LD 3 will vary depending on 
temperature. Therefore, if the bias current I.sub.B of the LD 3 keeps 
substantially constant regardless of the temperature, the extinction ratio 
and the peak value of the optical output may be kept constant. 
Furthermore, if a resistor having a positive and large temperature 
coefficient is used as the resistor 11, excess power consumption at a high 
temperature may be prevented from increasing substantially. It is 
currently considerable to use about 5000 ppm/.degree.C. of the temperature 
coefficient for the resistor. Accordingly in the temperature range of 
-30.degree. C.-70.degree. C., the resistance value at the temperature of 
70.degree. C. is 1.5 times as much as that at 30.degree. C. and the power 
consumed by the resistor 11 may be reduced to 67% in comparison with the 
apparatus excluding the compensation circuit 13. 
A fifth embodiment of the present invention is illustrated in FIG. 10. In 
FIG. 10, the circuits which are the same as those shown in FIG. 4 are 
denoted by the same numerals and numeral 14 designates a constant current 
source of which the current is set at a value less than the current value 
necessary for enabling the LD 3 to oscillate. 
Since an LD provides such optical output vs drive current and voltage vs 
drive current characteristics as shown in FIG. 11, be optical output may 
be provided at the current more than the oscillation threshold current 
value for example 10 mA as shown in FIG. 11, while the voltage between 
terminals of the LD may reach a normal voltage for example 1 V as shown in 
FIG. 11 by being supplied a small current, for example 1 mA as shown in 
FIG. 12. 
Accordingly, by selecting a suitable current in the range from 1 to 10 mA, 
it is understood that the LD will not provide the optical output but the 
voltage between its end terminals will be of a normal value. 
Therefore according to the apparatus shown in FIG. 10, if the current value 
of the current source 14 is selected to be in the order of 1.5 mA for 
example, the LD 3 will not provide the optical output when it is 
prohibited and the voltage between its end terminals will be in the order 
of 1 V, whereby the common mode input voltage of the monitoring circuit 10 
will drop by approximate 1 V from the voltage V.sub.CC of the source. 
Therefore, the output from the monitoring circuit will not be attracted by 
the voltage V.sub.EE of the other source and no erroneous alarm will be 
provided. 
FIG. 12 illustrates a sixth embodiment in which a switch 15 is added to the 
embodiment shown in FIG. 10 by connection in series to the current source 
14. The switch 15 is so designed as to be turned on by the optical output 
inhibit signal applied to the terminal 9. Accordingly in the normal 
operation, the current source 14 is separated from the LD 3 to reduce the 
consumption of electric power. 
In the case of the embodiments shown in FIGS. 10 and 12, if the current 
value of the current source 14 is set sufficiently low taking into 
consideration the possibility of the oscillation threshold current value 
of the LD 3 being reduced at a low temperature, the extinction ratio will 
never be degraded. If a resistor is connected in parallel with the LD 3, 
it is necessary to set the current value of the current source 14 at a 
rather large value, but the current value may be suitably set in 
conjunction with degradation of the extinction ratio. 
In the respective embodiments as described above, although a Darlington 
connection current amplifier is employed as the current amplifier 6, it is 
of course possible to employ a combination of a voltage amplifier and a 
voltage/current conversion circuit. 
It is also possible that the switch 52 (see FIGS. 10 and 12) provided for 
prohibition of the optical output may be connected in series to the 
resistor 101 of the monitoring circuit 10 to execute the same operation. 
As explained above, according to the first through third embodiments of the 
present invention, the current amplification ratio of the current 
amplifier may be kept at sufficiently high values at the respective 
temperatures and a favorable APC operation may be assured by connecting 
the resistor circuit in parallel with the LD. 
Furthermore, according to the fourth embodiment of the present invention, 
by adding the modulated current temperature compensation circuit adapted 
to compensate for temperature change and to control the value of the 
binary current from the current modulation circuit to the LD, the bias 
current for the LD may be kept constant in a wide temperature range, and 
the extinction ratio and the peak value of the optical output may be kept 
constant. 
According to the fifth and sixth embodiments of this invention, by 
connecting the constant current source in series to the LD in the optical 
transmission apparatus including the bias current monitoring circuit, the 
monitoring circuit will not provide any alarm output when the optical 
output is inhibited. 
It is further understood by those skilled in the art that the foregoing 
description represents preferred embodiments of disclosed device and that 
various changes and modifications may be made in the invention without 
departing from the spirit and scope thereof.