The temporal characteristics of Brillouin back scattering pulses from a single cell with focus geometry were investigated experimentally. The temporal variation of the phase conjugation fidelity and the shaping pulse with high contrast using two cells were also observed. The results show that the profile of the reflected pulse is closely related to pump energy and focal length of the lens. The pulse start point is not stable and the variation is about 1.5 ns. The pulse shaping using a multi cell scheme is practicable in principle.