1. Field of the Invention
The present invention relates to an optical signal transmitter using a laser diode. More particularly, the invention relates to an optical signal transmitter having an APC circuit with automatic bias current control.
2. Description of the Related Art
In an optical signal transmitter using a laser diode (LD) for directly modulating the diode with an input signal, a bias current and a threshold current are changed due to secular deterioration of LD characteristic, temperature variation and electric source change, which causes the problems of change in the LD light-emitting power characteristic and deterioration of transmitting characteristic.
Thus, various kinds of circuit have been proposed to keep the characteristic of light-emitting power of LD constant with an APC (Auto Power Control) circuit. FIG. 13 is a block diagram showing one example of the circuit for transmitting an optical signal, having an APC circuit.
In FIG. 13, a driving circuit 1 makes a LD driving current Ip from a first current source 2 flow to a LD (laser diode), corresponding to `1` or `0` of the input signal inputted from an input terminal IN, and switches the laser light-emission ON or OFF.
Further, a second current source 3 supplies a direct bias current Ib required to be input to the LD. An LD driving control voltage Vp and a bias control voltage Vb of the LD are supplied to the first current source 2 and the second current source 3, respectively.
On the other hand, a part of the laser light emitted from the LD is received by the photo diode (PD), and the average value is converted by a resistor R1 to a voltage Vpd in proportion to the laser light emitted from the LD.
An amplifier 4 controls the output voltage Va so as that the voltage Vpd becomes equal to the reference voltage Vref. Further, the above-described control voltages Vp and Vb are obtained by amplifying the output voltage Va on the amplifiers 5 and 6 having each gains of Gp and Gb.
It is controlled automatically, so as to keep the light output of the LD constant by the feedback loop. Further, the ratio of Ip to Ib is set by the gains Gp and Gb of the amplifiers 5 and 6, optionally.
Hereupon, it is a static characteristic of the LD with changed inclination .eta. and threshold current Ith, equally, according to the temperature as shown in FIG. 14. The light output (Pout) can be obtained from the LD, as shown in the diagram of FIG. 14, by setting the bias current Ib near around the threshold current Ith, and flowing the driving current Ip in addition to the bias current Ib.
Accordingly, it is required to keep the relation of Ib and Ith at the optimal value, in the cases where an optical signal is transmitted by switching LD ON or OFF, at high speed. In the prior art shown in FIG. 13, the gains Gb and Gp of the amplifiers 5 and 6 are controlled, to keep Ib/Ith on the optimal value.
Further, it is also required for keeping the light output and Ib/Ith constant to change the rate of Ip to Ib according to the temperature. However, the conventional structure does not have any means for changing the ratio of Ip to Ib, independently, according to the temperature.
Thus, there is a problem of deterioration of the transmitting characteristic, as a result of changing the relation of Ib/Ith depending on the temperature and not obtaining the optimal value.