To obtain a synthetic aperture ladar signal with a high net power rate, a novel method to obtain the Linear Frequency Modulation(LFM) laser signal was presented and the LFM laser experiment was also performed. Detailed Fourier analysis for the driving signal shows that the cavity tuning device can not response the high-order harmonics well. A high-order harmonics response module was introduced into the driving circuit of the cavity resonator to drive the piezoelectric transducer for cavity tuning, so that the trailing edge of the signal was shortened and the net power rate of the signal was improved greatly. Furthermore, the performance of obtained signal was tested and analyzed. The results show that when the FM period is 1 ms, the FM bandwidth is 789 MHz with the FM linearity better than 1.6%. Meanwhile, the net power rate of the signal is raised to 92% from 72%.