To improve a signal light-to-noise ratio by providing a light source that generates offset light, an optical coupler which combines signal light and the offset light and a saturable absorption type optical element which inputs an optical coupler output and outputs it to a receiving circuit.
Signal light 11 transmitted from a transmission line 4 is transmitted to a receiving circuit 2 via a saturable absorption type optical element 3 which suppresses the light noise of the light 11. The element 3 has a saturable absorption effect due to the reverse bias voltage application to a semiconductor waveguide and has a nonlinear transmitted characteristic depending on input light intensity. Although a data pulse having large light intensity exceeds the absorption capability of the optical element and is transmitted, a noise component having small light intensity receives strong absorption and is reduced. Because of this, a pulse waveform of a peak value which includes the noise component and exceeds the threshold levels of the transmitted characteristic and the absorption characteristic can be made to a pulse waveform that reduces the noise component.
