Electro-optical modulator which provides optical bias stabilization during operation

#1. Field of the Invention

#The invention relates to an optical transmitter comprising a combination of an electro-optical converter and an opto-electrical converter which are optically coupled, which combination is included in the negative feedback path of a control loop, the input of the electro-optical converter being coupled to the input of the negative feedback path and the output of the opto-electrical converter being coupled to the output of the negative feedback path. The control loop includes a control amplifier, the output of the negative feedback path being coupled to an input of the control amplifier and the output of the control amplifier being coupled to a first input of combining means whose output is coupled to the input of the negative feedback path. Modulation signal is applied to a second input of the combining means.

#2. Description of the Related Art

#Hereinbelow the invention will be further explained with reference to the drawing Figures, in which:

#FIG. 1 shows a block diagram of a transmitter according to the invention;

#FIG. 2 shows a gain control comprising a summator and an integrator to be used in a transmitter as shown in FIG. 1; FIG. 3 shows an automatic gain control 8, to be used in a transmitter as shown in FIG. 1;

#In FIG. 1 a reference signal b generated by a cancelling circuit 10 is fed to a first input of a control amplifier 3 constituted by a summator 1 and an integrator 2. The output of summator 1 is connected to the input of the integrator 2. The output of the integrator 2 is connected to a first input of a combining means which in this case is constituted by a summator 4. The output of the summator 4 is connected to the input of an electro-optical converter whose input is formed in this case by the anode of a laser diode 6. The cathode of the laser diode 6 is connected to a point of reference potential to be termed earth hereinafter. The laser diode 6 is optically coupled to an opto-electrical converter constituted by a photodiode 7. The cathode of the photodiode 7 is also connected to earth. The anode of the photodiode 7 is connected to the input of an automatic gain control amplifier 8. The combination of laser diode 6, photodiode 7 and automatic gain control amplifier 8 will be referred to as a cascade circuit 5. The output of the automatic gain control amplifier 8 is connected to the input of a low-pass filter 9 whose output is connected to a second input of the summator 1. The output of the automatic gain control amplifier 8 is also connected to the input of a band-pass filter 11. A negative feedback path of a control loop is formed by the cascade circuit 5, the low-pass filter 9 and the summator 1.

#An auxiliary signal h is fed to an input of an auxiliary circuit 13. A first output of the auxiliary circuit 13 carrying output signal h' is connected to a second input of the summator 4. A second output of the auxiliary circuit 13 carrying output signal y is connected to a first input of a comparator circuit 12. The output of the band-pass filter 11, carrying output signal x, is connected to a second input of the comparator circuit 12. The output of the comparator circuit 12, carrying output signal z, is connected to a control input of the inverting amplifier 8. A modulation signal m is fed to an input of the cancelling circuit 10. A first output of the cancelling circuit 10, carrying output signal m', is connected to a third input of the summator 4. A second output of the cancelling circuit 10, carrying output signal c, which signal forms the cancelling signal in accordance with the invention, is connected to an input of the summator 1.

#In FIG. 1 a control loop for fixing the bias of the amount of light emitted by the laser diode 6 is formed by the control amplifier 3, the summator 4, the cascade circuit 5 and the low-pass filter 9. The control loop maintains the value of the output signal of the negative feedback path at -b, which signal is constituted in this case by the output signal of the low-pass filter 9, as a result of which the signal at the output of control amplifier 3 produces a bias level of the amount of light U emitted by the laser diode 6 given by, ##EQU1## In (1) d is the light intensity-to-current intensity conversion factor of the photodiode 7 and A is the gain factor of the amplifier 8.