The parameters of light source in synthetic aperture ladar (SAL) is very important to both the design of system and the signal processing algorithm. As the light source in the SAL, the fiber laser uses PZT stretching the fiber Bragg grating in order to tune the laser frequency. So it is necessary to obtain the deformation and frequency response of PZT pipe driven by sawtooth voltage of different periods. Accordingly, the homodyne detection is used to measure the girth variation of PZT. Meanwhile ,the frequency response of PZT can be viewed with the homodyne signal. The results from measuring the PZT pipe show that the method can work well.