PURPOSE: To perform transmission with high efficiency and low distortion by providing a means which keeps the modulation factor of output light of a light emitting element constant for temperature change by using a monitoring output.
CONSTITUTION: The modulation component of a signal converted to an electric signal by a photodiode 2 for monitor is extracted by a high-pass filter 5, and a component in accordance with mean light power can be eliminated. A modulation signal component obtained at the filter 5 is detected and rectified at a modulation signal control circuit 6, and after that, it is compared with the reference voltage of the modulation component in accordance with a certain modulation factor, and the gain or the modulation factor of a variable gain amplifier 7 is controlled based on a comparator error voltage at the control circuit 6. Thereby, the light output modulation factor of a light transmission circuit can be kept constant with the mean light output, and an optical transmission signal with satisfactory efficiency and low distortion can be obtained.
SAKANAKA TETSUO
KONNO HARUO
IDEKURA SEIZABUROU
