An all-optical sampling scheme using a single semiconductor optical amplifier is proposed for analog signal processing. The analog optical signal is sampled by the probe pulse train through the nonlinear polarization rotation arising in the semiconductor optical amplifier. Conversion efficiency and total harmonic distortion are presented to evaluate the sampling linearity. In the experiment, 40 GSa/s all-optical sampling for 2.5 GHz analog optical signal is successfully demonstrated with commercially available fiber-pigtailed components. The results show that the fundamental conversion efficiency and the total harmonic distortion are 1.35 and 2.01% at the operating power of 5 mW, respectively. The proposed all-optical sampling requires only one semiconductor optical amplifier and has low power consumption, which is simple and has potential for photonic integration.