A novel optical ultrawideband (UWB) pulse generator, which is capable of generating both Gaussian monocycle and Gaussian doublet pulses, by using a directly modulated semiconductor laser, optical filtering and chromatic dispersion in fiber is proposed and experimentally demonstrated. The combination of directly modulated laser and optical filter performs an electro-optic derivative operation, which produces a monocycle pulse when Gaussian pulse driving current is applied. The transmission of the obtained signal in single mode fiber is also considered, the chromatic dispersion has strong influences on the laser chirped pulses. We exploit this accumulative dispersion in order to switch the pulse polarity and shape. The proposed system is implemented, and the generation of UWB monocycle and doublet pulses is experimentally demonstrated. Gaussian monocycle and doublet pulses with a fractional bandwidth of about 137% and 108% are experimentally generated.