Abstract: For the laser driver circuit driving a laser diode to generate an optical output corresponding to an input data, thus, controlling the optical output based on an output current from a photodiode that detects the optical output. The output current is converted into a voltage signal whose amplitude is detected after filtering the voltage signal by a low-pass filter. Therefore, a reference signal corresponding to the input data is generated and its amplitude is detected. We have a modulator for generating a modulation current, it regulates the amplitude of the modulation current, according to the difference between the two detected amplitudes.