To acquire the instantaneous velocity outside the chamber of a high-speed bullet and its velocity distribution at different positions

this paper proposed a velocity measurement system with expanded beams based on the laser Doppler effect. The system expanded the exiting laser beam for two times and then made the two coherent beams intersect in the space to form a new probe volume with two dimensional broadening lengths. The system efficiently overcame the object deviation from the measurement region caused by the smaller probe volume in the traditional Laser Doppler Velocimiter (LDV). Therefore

it could measure the velocity of high-speed bullet that traditional LDV could not implement because of the trajectory deviation. Moreover

the system could also monitor the velocity distribution of the high-speed bullet at different positions in the trajectory by a parallel mode. Experiments for expanding different lasers were performed in different illuminating lights

and the Doppler signals were obtained from the photo-detector finally. The analysis results show that the average error of the measured velocities at 10-25 m/s can reach 1%-2%

which indicate that the proposed method is effective and feasible.