In a laser scanning imaging system, the modulation of light output is directly achieved by modulating the drive current of a laser diode input signal. So far, what kind of modulation waveform is the best remains undetermined. People usually choose sine modulation for convenience. Through our theoretical analysis and experiment, we have reached the conclusion that for the same receiver, substituting low duty (a duty of 14is small enough ) rectangular modulation for conventional sine modulation will offer at least a 5 dB improvement in S/N ratio with the average incident optical power unchanged.