Optical transmitters

An optical transmitter including a laser diode and achieving automatic slope variation compensation of the laser diode, the transmitter further including a photodetector disposed to detect the output optical signal of the laser diode, a first control loop and a second control loop, the first control loop serving to maintain the mean optical signal power output from the laser diode at a first predetermined level by adjustment of bias current applied thereto, in response to comparison of a voltage signal representing mean optical signal power obtained from the photodetector a.c. output signal with a voltage corresponding to the first predetermined level, and the second control loop serving to adjust modulation current applied to the laser diode whereby to maintain the a.c. optical signal power output from the laser diode at a second predetermined level, which modulation current adjustment is carried out in response to the detection of an error signal between the second predetermined level and a mean value of the a.c. optical signal power obtained in the second control loop from the photodetector a.c. output signal by successive amplification and rectification.