A new theoretical model is presented to investigate the amplification of strong ultrashort optical pulses in semiconductor optical amplifiers, while taking free-carrier absorption, stimulated emission, two-photon absorption, spectral hole burning and ultrafast nonlinear refraction into account. Based on this model, the pulsed four-wave mixing model is developed to further simulate the optical sampling based on four-wave mixing theory in semiconductor optical amplifiers. The influences of free-carrier absorption and two-photon absorption on sampling characteristics are emphasized. The simulation results are in agreement with reported experimental results.