The nonlinear chirp of tunable laser in synthetic aperture imaging ladar (SAIL) can degrade the range resolution and influence the azimuth resolution. The polynomial nonlinear coefficients of laser frequency changed with time are measured by a fiber Fabry-Perot interferometer. The results of experiment are presented. Based on the data the Fourier frequency spectrum of the point target is discussed on the condition of the different frequency linearity. The spectral width and phase errors are reduced through changing the sampling pulse width. It is one method to compensate the degradation of resolution. The optimized sampling pulse width is derived at the phase error better than π/4 corresponding to the optical diffraction limit.