We report on a theoretical analysis of the dynamical performance of a single-frequency semiconductor laser under the influence of nonlinear optical feedback. It is shown that the modal structure of such a laser is both dependent on the mirror characteristics and on the pump current. The analysis concentrates on the self pulsating regimes that can be observed, and more precisely on the occurrence of harmonic passive mode-locking. Generation of short pulses, characterized by a duration around 10 ps and a repetition rates as high as 20 GHz, have been demonstrated. The characteristics of this mode-locking strongly depend on the nonlinear mirror's temporal response.