OPTICAL TRANSMITTER

PURPOSE:To obtain a high-accuracy light output and a high-accuracy extinction ratio by a method wherein the temperature of a laser diode using a thermocooler is controlled by the wavelenght of light, which is generated from the laser diode, and the light level. CONSTITUTION:A light output of a laser diode 3 is branched by a light branching device 7 in the direction of measurement, a selection of the wavelength of light, which is generated from the diode 3, is performed by a spectroscope consisting of a diffraction grating and the like, the light level is converted into an electrical signal and the electrical signal is outputted. This electrical signal is averaged by a first average arithmetic device, the voltage of the averaged electrical signal is compared with a previously set reference voltage of a first threshold voltage generator 11 by an operational amplifier 10 and a difference voltage between both voltages is taken out. A current driver 12 increases and decreases a current of a thermocooler 13 using this difference voltage as the input, the temperature of the diode 3 is held at a constant temperature and each current of a modulator 2 and first and second bias current sources 14 and 15 is set. Thereby, even if current-light conversion characteristics to the temperature of the laser diode are fluctuated, a constant light output and the constant wavelength of light are obtained.