Synchronized dual-wavelength 1-MHz repetition-rate optical pulses were successfully generated by the combination of a gain-switched diode laser and a wavelength-tunable continuous-wave diode laser at 1.5 mum. The timing synchronization of dual-wavelength optical pulses was achieved with four-wave mixing by use of a highly nonlinear optical fiber. This optical pulse source was utilized for terahertz (THz)-wave differencefrequency generation by use of slanted periodically poled LiNbO3 (PPLN). We generated between 1.05 and 2.1 THz by use of the proper grating period of slanted PPLN with a 10-GHz bandwidth.