A Nd:YAG laser oscillator Q-switched and mode locked via a nonlinear mirror based on stimulated Brillouin scattering (SBS) and aided by an acousto optical modulator (AOM) has been realised. A rate equation model and a round trip model are used to describe the longitudinal mode dynamics. Parameter variations of the nonlinear SBS mirror and the loss modulating AOM in the resonator are investigated with regard to their effect on the pulse duration and spectra of the pulses.