A system is reported in which a semiconductor optical amplifier is used to generate laser signal. The system mainly uses a terahertz laser which can generate two different frequencies and whose frequencies can be adjusted. The optical signal generated by the laser is converted into a stable electrical signal by a photodiode. As a low-noise radio frequency source, electrical signal can generate ultra-short pulse signal with low jitter and high repetition rate. Then, theoretical analysis and simulation experiment are carried out for each part of the system. According to Gaussian pulse theory, the parameters of pulse function in steady state are obtained. Finally, the single pulse and continuous pulse with different modulation frequency are simulated. The change of pulse is analyzed by changing the value of each parameter. The simulation results show that high repetition rate continuous ultrashort optical pulses can be obtained in the range of 10-30 GHz.