We propose an ultra-wideband monocycle signal source based on the cross-gain modulation(XGM) in a semiconductor optical amplifier(SOA).In the scheme,only an optical source and a SOA are needed.So this scheme is simple.As the opposite working way is used,the scheme has advantages in improving the ER of the output signal.There is only one wavelength included in the output monocycle single pulse,so no time difference between the upper and down pulses is introduced during the transmission.By using t...