The present invention relates to none return to zero (NRZ) modulation method. The NRZ optical modulation is performed by combining a clock signal and NRZ data at a sending end and signal distortion capable of being generated when the clock signal and the NRZ data are combined is optimized by controlling the magnitude and phase of the clock signal. At the receiving end, the clock signal is extracted by performing narrow band band-pass filtering of the detected optical signal transmitted from a transmitter and data is recovered using the clock signal. Therefore, a receiver structure for clock extraction is simpler, an error rate of data recovery is lower by clearly extracting the clock signal, and transmission distance of the optical signal is longer.